Arrangement and a method for coupling light into a plate-like light guide

ABSTRACT

An arrangement ( 20 ) for coupling light into a plate-like guide ( 25 ) having two surfaces ( 23, 24 ) on opposite sides of the light guide comprises an in-coupling diffraction grating ( 21 ) for diffracting an external light beam ( 26 ) incident on said in-coupling diffraction grating into the light guide in a direction enabling the in-coupled light beam to propagate within the light guide via total internal reflections at the light guide surfaces ( 23, 24 ). According to the present invention, the arrangement further comprises a deflection member ( 22 ) arranged to deflect the beam ( 27 ) initially diffracted by the in-coupling diffraction grating ( 21 ), before it hits the in-coupling diffraction grating again, out of the path determined by the in-coupling diffraction grating in order to reduce out-coupling of the already in-coupled light through the in-coupling diffraction grating.

FIELD OF THE INVENTION

The present invention relates to coupling light into optical lightguides, more particularly to coupling arrangements utilizing diffractiveoptics for coupling light into thin plate-like light guides.

BACKGROUND OF THE INVENTION

There are a number of applications where effective coupling of lightinto a light guide is desired. Some examples include different kinds ofbacklights and virtual displays designed to re-distribute lightoriginally emitted e.g. by a LED (Light Emitting Diode).

A traditional approach is to feed the light into a plate-like lightguide through its edge which gives a high coupling efficiency especiallyin the case of a light guide thicker than the beam size. However, inmany applications it is the case that the thickness of the light guideis smaller than the size of the light beam to be coupled in. In thesecases it is desirable to couple the light through the top or bottomsurface of the light guide. The simplest solution is to use adiffraction grating on the side of incoming light of the light guidewith a grating period suitable for in-coupling. This method isapplicable as long as the lateral dimensions of the grating are up toabout twice as large as the light guide thickness. In the case ofreduced light guide thickness, the in-coupled light hits the gratingarea again after reflection at the opposite side of the light guide.Then, due to the reverse propagation of light, it is again coupled out,at the worst with the same efficiency as it was originally coupled in.This leads to great losses resulting in a poor total in-couplingefficiency. Thus, new optical designs must be used for achieving asufficiently efficient coupling.

As one approach intended to avoid the problems described above, it isknown to use radial grating geometry for a non-collimated LED light.With this kind of grating it has been possible to reduce the light guidethickness to 0.6 mm and this solution has also been successfullyexpanded to white light applications. However, in this kind ofarrangement, the in-coupled light beam disperses in all directions inthe plane of the waveguide without any possibility to confine itspropagation in some particular direction.

Levola discloses in patent application US2005/0002611 A1 a structure forcoupling light into a wave guiding substrate, the structure including apolarization converting element for changing the polarization state ofthe incoming light after the first interaction with the in-couplinggrating. According to the description, changing the polarization from TEto TM between the first and the second interactions with the in-couplinggrating makes it possible to design said grating so as to decreasereverse diffractions of the in-coupled light out from the light guide.As a result, the overall coupling efficiency is improved. It is reportedthat the coupling area can be made about two times wider than withoutthe polarization element. However, an essential drawback of thissolution is the limitation to specific polarization states of the light.

In solutions disclosed in patent publications JP11174270 and JP11281833,the coupling grating on a first surface of a waveguide formed on asubstrate divides the incident light beam into two sub-beams propagatingwithin the light guide with different directions. The basic idea of thedisclosed solutions is that one of said sub-beams follows such a paththat, after having been reflected at the second surface of the waveguideor from a buffer layer between the waveguide and the substrate, it isphase matched with another incident light beam and is coupled with itvia an interferences phenomenon.

DE 4131738 A1 discloses an arrangement for in-coupling or out-couplinglight between a waveguide and the ambient, the arrangement comprisingtwo gratings. The waveguide has been formed on a glass plate substratesubstantially thicker than the waveguide itself. There is a firstgrating on the surface of the waveguide guide serving as a redirectingelement making light propagating within the waveguide diffract to thesubstrate. The light beam redirected or diffracted by the first gratinghas a predetermined intensity distribution. On the opposite surface ofthe substrate there is another grating serving as a phase correctingelement correcting the phases of the light rays of the beam escaping thestructure. The disclosed arrangement works also reversely forin-coupling. However, due to the thick substrate necessitated and theactually rather complex grating design needed to execute the functionsdescribed above, this particular approach is far from a generallyapplicable solution for coupling light into thin light guides.

PURPOSE OF THE INVENTION

The purpose of the present invention is to provide a novel solution forefficiently coupling light into thin light guides without the defectsand limitations of the prior art techniques.

SUMMARY OF THE INVENTION

The arrangement and method for coupling light into a plate-like lightguide according to the present invention are characterized by what ispresented in claims 1 and 11, respectively.

In the most typical applications, the light to be coupled into the lightguide is in the range of visible wavelengths. However, the principle ofthe present invention is basically not limited to any specificwavelength and it could be exploited in case of ultraviolet or infraredlight as well.

The plate-like light guide has two surfaces on opposite sides of thelight guide. Plate-like means here a structure having its lateraldimensions substantially greater than the thickness of the light guide.This kind of light guide is often substantially planar but there can bealso applications where at least one of the light guide surfacescomprises an area which is slightly curved and/or tilted with respect tothe surface on the opposite side of the light guide. It is also possiblethat the entire light guide is slightly bent.

The arrangement comprises an in-coupling diffraction grating fordiffracting an external light beam incident on said in-couplingdiffraction grating into the light guide in a direction enabling thein-coupled light to propagate within the light guide via total internalreflections at the light guide surfaces. The in-coupling diffractiongrating can be a transmission type grating located near the light guidesurface on the side of the incoming light. It is also possible that thein-coupling diffraction grating lies near the light guide surface on theopposite side of the light guide and is of reflection type, beingpossibly metallized. Being diffracted in said direction enabling totalinternal reflections at the light guide surfaces means that when thein-coupled light leaves the in-coupling area of the arrangement,typically after several interactions with the in-coupling diffractiongrating and the opposite surface of the light guide as well as possibleother optical elements in that area, it continues propagation within thewaveguide without escaping it through the light guide surfaces. As isclear for a person skilled in the art, a direction in which totalinternal reflection at the light guide surface is possible depends onthe difference in the refractive indices between the light guide and theambient. It is of standard procedure for a professional to design suchan optical arrangement that said condition of total internal reflectionsis fulfilled.

According to the present invention, the arrangement further comprises adeflection member arranged to deflect the light beam initiallydiffracted by the in-coupling diffraction grating, before it hits thein-coupling diffraction grating again, out of the path determined by thein-coupling diffraction grating. Said path determined by the in-couplingdiffraction grating comprises naturally the initial direction of thein-coupled light beam, but includes also the propagation direction ofthe in-coupled light after a reflection at a light guide surfacecoplanar with the in-coupling diffraction grating. The purpose of thedeflection member is to reduce cut-coupling of the already in-coupledlight through the in-coupling diffraction grating when hitting it again.Deflecting out of said path means that the deflected beam is no more inthe direction of effective function of the in-coupling diffractiongrating. Thus, the reverse coupling out from the light guide through thein-coupling diffraction grating is inefficient, thus leading to highlyincreased total coupling efficiency in comparison to a conventionalarrangement based on one single grating only.

Depending on the actual design of the arrangement and the deflectionmember type, the deflection member can be arranged to deflect the beaminitially diffracted by the in-coupling diffraction member in adirection out of and/or in a plane defined by the directions of theincident and the initially diffracted light beams.

Preferably, the deflection member comprises a deflecting diffractiongrating. By means of a diffraction grating, the desired deflection canbe realized effectively and accurately. For example, in the case of thein-coupling and deflecting diffraction gratings each consisting ofstraight and parallel grating lines and being placed on the coplanaropposite sides of the light guide, the deflection can be implementedsimply by having the grating geometries being rotated, e.g.perpendicularly, with respect to each other. In addition to or insteadof a diffraction grating, the deflecting member can also comprisedifferent kinds of tilted facets or other optical elements providingeffects other than diffraction.

In one preferred embodiment, at least one of the in-coupling diffractiongrating and possible deflecting diffraction grating is a surface reliefstructure formed on one of the surfaces of the light guide. From amanufacturing point of view, a surface relief structure is the moststraightforward approach. However, for example, for enabling mechanicalwashing of the surface of the light guide, at least one of the gratingscan be formed as a buried structure within the light guide. This meansthat the grating is not the outermost layer of the light guide structurebut there is an additional layer of the light guide body material orsome other material on the grating. A buried grating structure can alsobe formed in a material differing from those of the traditional lightguide materials and having possibly a higher refractive index.

Preferably, the in-coupling diffraction grating is an asymmetricgrating, for example a slanted a blazed grating, in order to confine thedispersion of the in-coupled light in the direction of the plane of thelight guide. An asymmetric grating means here a grating providingdiffraction efficiently into only one of the positive and negativediffraction order(s). For example, with a simple arrangement comprisingperpendicularly located symmetric grating geometries, each consisting ofparallel grating lines, the in-coupled light basically propagates in atleast four gradually diverging cones which might be undesired in manyapplications. The confining effect can be maximized by choosing also thepossible deflecting diffraction grating to be of asymmetric type. Insuch a case it is possible to provide only one direction of propagationin the plane of the light guide, provided that the both gratingsdiffract light into one diffraction order only.

In general, the present invention is not limited to any particulargrating type but the type and the grating parameters can be selectedfreely to meet the requirements of the actual application. In additionto the basic case of collimated light of a single wavelength, thepresent invention is also suited for a partly diverging incident beamand for light consisting of several wavelengths. Designing thegrating(s) to produce the desired effect(s) can be done bycomputer-aided calculation tools with procedures well known for thoseskilled in the art.

In one preferred embodiment, the in-coupling diffraction grating and thedeflecting member both are optimized to maximize the total diffractionefficiency for unpolarized incident light. This means that the functionof the arrangement is not dependant on some specific polarization stateof the incoming light. This maximizes the versatility of the arrangementNaturally, it is also possible to optimize each of the in-couplingdiffraction grating and the deflecting member for any polarization statein accordance with the requirements of the actual application. Again,optimizing the gratings for unpolarized or for some specificpolarization is of standard routines for those familiar with thediffractive optics. Thus, no detailed description about how to do it isneeded here.

In a preferred embodiment, the thickness of the light guide is less thanor equal to 1 mm, preferably less than or equal to 0.5 mm, mostpreferably less than or equal to about 0.05 mm. Already thicknesses like1 or 0.5 mm provide significant improvements in the coupling efficiencywhen compared to the prior art solutions. In the case of incident lightbeing in the form of a very narrow laser beam, suitable thickness of thelight guide can be even as low as 0.05 mm. In such a case the advantagesof the present invention over the prior art solutions are revolutionary.

In one preferred embodiment, the width of the in-coupling diffractiongrating in the direction of the projection of the in-coupling grating'sdesigned diffraction direction in the plane of the light guide is atleast 4 times, preferably at least 8 times, for example 16 times as bigas the thickness of the light guide. These kinds of widths together witha high in-coupling efficiency are not possible with conventional singlegrating systems. In the simple case of a grating consisting of straightgrating lines, said direction defined above simply means the directionperpendicular to the grating lines.

The method for coupling light into a plate-like light guide having twosurfaces on opposite sides of the light guide comprises a step ofdiffracting, by means of a diffraction grating, an external light beamincident on said diffraction grating into the light guide in a directionenabling the in-coupled light to propagate within the light guide viatotal internal reflections at the light guide surfaces. According to thepresent invention, the method also comprises step of deflecting thelight beam initially diffracted by means of the in-coupling diffractiongrating, before it hits the in-coupling diffraction grating again, outof the path determined by the in-coupling diffraction grating in orderto reduce out-coupling of the already in-coupled light through thein-coupling diffraction grating. Deflecting out of said path means hereany way of changing the propagation direction of the in-coupled lightinto a direction differing from a direction of reflection from a planecoplanar with the diffraction grating. Deflecting can be performedthrough, for example, diffraction, reflection or refraction by means ofa diffraction grating or some other optical element, respectively, or acombination thereof.

In one preferred embodiment, at the step of deflecting the light beaminitially diffracted by means of the in-coupling diffraction grating,said light beam is deflected out of a plane defined by the directions ofthe incident and the initially diffracted light beams. On the otherhand, the initially in-coupled light beam can also be deflected in adirection within said plane.

To summarize, the present invention provides numerous essentialadvantages over the prior art solutions. The in-coupling area of thearrangement can be far larger than in the conventional solutionssuffering from undesired out-coupling in cases the incident light beamis substantially wider than the thickness of the light guide. Thearrangement and method according to the present invention are notlimited to any specific polarization state of the light and the basicinventive idea can be applied also to non-coalmated and broadband light.The grating and deflection member types can be chosen freely to meet theactual requirements of the application at issue. A structure accordingto the present invention can be processed with standard manufacturingtechniques suitable for cost-effective mass-production without anyparticular and complex extra steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, which are included to provide a furtherunderstanding of the invention and constitute a part of thisspecification, illustrate embodiments of the invention as well as priorart examples and, together with the description, help to explain theprinciples of the invention.

FIG. 1 shows the principle of a typical application of coupling lightinto a thin light guide.

FIG. 2 represents a prior art coupling arrangement.

FIG. 3 shows an arrangement for coupling light into a light guideaccording to one preferred embodiment of the present invention.

FIG. 4 illustrates the operation of the arrangement of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an optical device 1 having an in-coupling region 2 and anout-coupling region 3 arranged on a thin light guide 4 is shown. Thepurpose of the in-coupling region is to diffract light 5 incident on itinto the light guide in a direction which allows the diffracted light topropagate within the light guide via total internal reflections at thesurfaces 6, 7 of the light guide. Correspondingly, the intended functionof the out-coupling region is to couple the light propagating within thelight guide again out of it. This kind of arrangement can be used e.g.in virtual displays or different kinds of illumination applications.Besides the basic configuration of FIG. 1, in some applications therecan be several cut-coupling regions around the in-coupling region.

The main problem related to arrangements shown in FIG. 1 is that therehas not been available a solution for effective in-coupling in caseswhere the light guide thickness is substantially smaller than thedesired width of the diffractive in-coupling region. This problem can beunderstood from what is shown in FIG. 2 where a cross section of a priorart arrangement for in-coupling is shown. The arrangement comprises alight guide 8 on the upper surface 12 of which a binary type diffractiongrating 9 has been formed. The diffraction grating is designed todiffract an incident light beam 10 into the light guide as diffractedbeams 11 with directions allowing total internal reflections at thelight guide surfaces 12, 13. If the area of the diffraction grating iswide enough, this kind of diffracted beam can hit the diffractiongrating again after the first reflection at the bottom surface 13 of thelight guide. Due to the reverse propagation of light, part of this beamis then coupled out of the light guide with an efficiency the same asthat of the initial in-coupling. With an asymmetric type grating, theout-coupling could be less efficient but basically the problem stillwould exist. For example, with a slanted type diffraction grating, thelight hitting the grating again would be reflected backwards and escapethe structure along the same path it came in. The out-coupling meansthat the total in-coupling efficiency remains low. In practice,depending on the grating size and the light guide thickness, couplingefficiencies exceeding 20% are very difficult to achieve by this kind ofarrangement. It is to be noted that the drawing of FIG. 2 is not inscale. The real dimensions of the grating line cross-sections typicallyare in a range of hundreds of nanometers while the light guide thicknesscan be, for example, about 0.5 mm. This is the case also for thedrawings in the other figures.

A part of an in-coupling arrangement 20 according to the presentinvention is shown in FIG. 3. Contrary to the prior art solutions basedon one binary type diffraction grating, there are two asymmetric slantedtype diffraction gratings 21, 22 at the opposite surfaces 23, 24 of alight guide 25. In this example, the gratings are arrangedperpendicularly to each other. The first one 21 of the gratings isdesigned to diffract incident light into the light guide so that it canpropagate within the light guide via total internal reflections at thesurfaces 23, 24 of the light guide. The other diffraction grating 22 onthe opposite surface 24 of the light guide is intended for deflectingthe initially in-coupled light sideways so that when the light beam hitsthe in-coupling diffraction grating again coupling out through it willbe inefficient. Both of the gratings are designed to provide effectivediffraction in the first diffraction order only. In this kind ofembodiment where the gratings consist of simple straight grating lines,the parameters for each grating to be chosen in the design phase caninclude, for example, the grating period d_(i), the width c_(i) of theridges of the grating, and the grating depth h_(i) (i=1, 2). Otherimportant parameters are the thickness L of the light guide and thewidth w of the in-coupling diffraction grating in the directionperpendicular to its grating lines.

The operational principle of the arrangement of FIG. 3 is illustrated inFIG. 4. An incident light beam 26 is firstly diffracted at thein-coupling grating 21 into the light guide in a direction according tothe first diffraction order. This initially diffracted beam 27, whenhitting the deflecting diffraction grating 22, is reflectivelydiffracted in a direction according to the first diffraction order ofthe deflecting grating. Due to the rotated position of the deflectinggrating with respect to the in-coupling grating, this second diffractionmakes the resulted beam 28 to be deviated sideways from a plane definedby the directions of the incident beam 26 and the initially diffractedbeam 27. Thus, when interacting with the in-coupling grating 21 again,the light is no more in the direction of optimal function of thatgrating and is therefore not coupled out through it but is reflecteddownwards again. When designing this kind of arrangement, it needs to betaken into account that, when the angle φ of the direction ofpropagation with respect to said plane defined above is altered, alsothe angle θ with respect to the normal of the plane of the light guidechanges. Thus, it must be ensured that, when finally exiting thein-coupling area of the light guide, the in-coupled light stillpropagates in a direction which enables total internal reflections atthe light guide surfaces.

The performance of the present invention has been proved by simulations.For example, a simulation was performed for a very simple two-gratingsystem with the in-coupling grating and the deflecting member beingbinary type gratings consisting of straight and parallel grating lines.The grating geometries were placed perpendicularly to each other assurface relief structures on the top and bottom surfaces of a 0.75 mmthick planar light guide having a refractive index of 1.50 for visiblewavelengths. For the simulation, the in-coupling grating was designedwith the main target being to produce maximum diffraction efficiency tothe first transmitted diffraction orders for TE-polarized light. Astraightforward optimization routine produced a grating periodd₁/λ=0.96, a grating fill-factor c₁/d₁=0.38 (material), and a gratingdepth h₁/λ=0.63 where λ is the wavelength of the light to be coupled in.For the deflection grating, a condition of keeping the beam propagationangle θ in relation to the normal of the light guide below 52° fixed thegrating period to d₂/λ=1.79. Then the grating on the bottom surface wasoptimized so as to have maximum reflected diffraction efficiency to thefirst diffraction orders. After the optimization, the followingparameters were obtained: c₂/d₂=0.35 (material) and h₂/λ=0.30. It isnoticeable that this bottom side grating is almost polarizationindependent for all the propagating rays. A ray trace analysis for thearrangement described above with grating sizes of 6×6 mm² and theincident beam hitting perpendicularly to the center of the couplingregion, resulted in a total in-coupling efficiency of as high as 59% forinput TE-polarization. This is far beyond the efficiencies normallyachievable with corresponding single grating systems. In general,different analyses have shown that the two-grating arrangement usuallyprovides efficiency at least about twice as high as that of a similarsystem with one grating only. The superiority of the present inventionis even emphasized as the light guide thickness is decreased.

With reference to the examples described above, it is to be noted thatthe grating parameters need not be constant over the grating area.Instead, they may vary according to the incident light properties andthe coupling performance desired. Neither need the grating lines bestraight. For example, if the light is desired to propagate within thelight guide in all directions, the in-coupling grating can consist ofcircular grating lines whereas the deflecting grating is then preferablyformed to have a radial grating geometry. In addition to surface reliefstructures shown in the figures, one or both of the gratings could beembedded within the light guide. The in-coupling grating can also be ofreflection type and located on or near the opposite side of the lightguide to the side of incoming light. And as was stated earlier in thisdocument, the deflection can also be implemented by some optical elementbased on some other effect than diffraction, e.g. reflection and/orrefraction. To summarize in general, it is obvious for a person skilledin the art that the basic idea of the invention may be implemented invarious ways. The invention and its embodiments are thus in no waylimited to the examples described above but they may vary within thescope of the claims.

The invention claimed is:
 1. An arrangement for coupling light into aplate-like light guide having two surfaces on opposite sides of thelight guide, the arrangement comprising: an in-coupling diffractiongrating for diffracting an external light beam incident on saidin-coupling diffraction grating into the light guide in a directionenabling the in-coupled light to propagate within the light guide viatotal internal reflections at the light guide surfaces, wherein thein-coupling diffraction grating is arranged on, within or near one ofthe light guide surfaces; a deflection member arranged to deflect thelight beam initially diffracted by the in-coupling diffraction grating,before it hits the in-coupling diffraction grating again, out of thepath determined by the in-coupling diffraction grating in order toreduce out-coupling of the already in-coupled light through thein-coupling diffraction grating, wherein the deflection member isarranged on, within or near the other one of the light guide surfaces,and wherein the path includes the initial direction of the in-coupledlight beam and the propagation direction of the in-coupled light beamafter an initial reflection at the other one of the light guidesurfaces.
 2. An arrangement according to claim 1, wherein the deflectionmember is arranged to deflect the light beam initially diffracted by thein-coupling diffraction grating in a direction out of a plane defined bythe directions of the incident and the initially diffracted light beams.3. An arrangement according to claim 1, wherein the deflection member isarranged to deflect the light beam initially diffracted by thein-coupling diffraction grating in a direction along a plane defined bythe directions of the incident and the initially diffracted light beams.4. An arrangement according to claim 1, wherein the deflection membercomprises a deflecting diffraction grating.
 5. An arrangement accordingto claim 4, wherein at least one of the in-coupling diffraction gratingand the deflecting diffraction grating is a surface relief structureformed on one of the light guide surfaces.
 6. An arrangement accordingto claim 4, wherein at least one of the in-coupling diffraction gratingand the deflecting diffraction grating is a buried structure formedwithin the light guide.
 7. An arrangement according to claim 1, whereinthe in-coupling diffraction grating is an asymmetric grating in order toconfine the spreading of the in-coupled light in the direction of theplane of the light guide.
 8. An arrangement according to claim 1,wherein the in-coupling diffraction grating and the deflection memberare optimized to maximize the total in-coupling efficiency forunpolarized incident light.
 9. An arrangement according to claim 1,wherein the thickness (L) of the light guide is less than or equal to 1mm.
 10. An arrangement according to claim 1, wherein the width (w) ofthe in-coupling diffraction grating in the direction of the projectionof the in-coupling grating's designed diffraction direction in the planeof the light guide is at least 4 times as big as the thickness (L) ofthe light guide.
 11. A method for coupling light into a plate-like lightguide having two surfaces on opposite sides of the light guide, themethod comprising: a step of diffracting, by means of an in-couplingdiffraction grating, an external light beam incident on said in-couplingdiffraction grating into the light guide in a direction enabling thein-coupled light to propagate within the light guide via total internalreflections at the light guide surfaces, wherein the in-couplingdiffraction grating is arranged on, within or near one of the lightguide surfaces; a step of deflecting, by means of a deflection member,the light beam initially diffracted by means of the in-couplingdiffraction grating, before it hits the in-coupling diffraction gratingagain, out of the path determined by the in-coupling diffraction gratingin order to reduce out-coupling of the already in-coupled light throughthe in-coupling diffraction grating, wherein the deflection member isarranged on, within or near the other one of the light guide surfaces,and wherein the path includes the initial direction of the in-coupledlight beam and the propagation direction of the in-coupled light beamafter an initial reflection at the other one of the light guidesurfaces.
 12. A method according to claim 11, characterized in that, atthe step of deflecting the light beam initially diffracted by means ofthe in-coupling diffraction grating, said light beam is deflected out ofa plane defined by the directions of the incident and the initiallydiffracted light beams.
 13. A method according to claim 11, wherein atthe step of deflecting the light beam initially diffracted by means ofthe in-coupling diffraction grating, said light beam is deflected in adirection along a plane defined by the directions of the incident andthe initially diffracted light beams.
 14. An arrangement according toclaim 1, wherein the thickness (L) of the light guide is less than orequal to 0.5 mm.
 15. An arrangement according to claim 1, wherein thethickness (L) of the light guide is less than or equal to about 0.05 mm.16. An arrangement according to claim 1, wherein the width (w) of thein-coupling diffraction grating in the direction of the projection ofthe in-coupling grating's designed diffraction direction in the plane ofthe light guide is at least 8 times as big as the thickness (L) of thelight guide.